Silicone copolymers serve as surface-tension lowering agents in agricultural adjuvants, stabilizers for polyurethane foam, additives for coatings applications, antifoams, and emulsifiers. The efficient manufacture of silicone copolymers is desired for two primary reasons--lower cost, and less waste. If, in addition, the equipment needed for that method or process is less costly to construct, such method or process would be inherently attractive. Moreover, there remains a need for a process to prepare silicone copolymers that provides desirable properties in the application for which they are intended, which process would offer manufacturers the flexibility to produce variations of products, determined by the choice of method of manufacture.
Chemical reactions may be conducted in a batch fashion, in a continuous fashion, or in hybrid fashion partially batch or partially continuous). For example, the reactants necessary to prepare a silicone copolymer may be a silicone fluid containing one or more hydrogen atoms directly bonded to silicon (hereafter referred to as a hydrogen siloxane or SiH fluid); and an olefinically terminated compound (hereinafter referred to as an olefinic compound). The two components are mixed together in appropriate amounts, and while being sufficiently agitated, catalyst is added. A vigorous reaction ensues, and the olefin, by hydrosilation, becomes chemically attached to the silicone.
Because in most cases the hydrogen siloxane fluid and the olefinic compound are immiscible, a compatibilizing agent frequently is used to facilitate reaction. This agent is often called a solvent, although it is not necessary to use it in sufficient quantity to dissolve both components. If the hydrogen siloxane fluid and olefinic compound are sufficiently pure of minor-to-trace components, the amount of "solvent" can be decreased, in some cases to zero. However, in those cases, good agitation becomes even more important, to maximize the contact between the two (relatively) immiscible phases.
The reaction between the raw materials need not be conducted in a purely batch fashion. For example, if the reactivity of the hydrogen siloxane fluid is very high, the olefinic compound may be charged to the reactor in its entirety, a fraction of the hydrogen siloxane fluid may be charged, the reaction catalyzed by adding a noble metal catalyst solution, and the remaining hydrogen siloxane fluid added subsequently and at such a rate, after the initial reaction exotherm has begun to subside, as to keep the reaction under control. This process is sometimes called semi-batch, or (incorrectly) semi-continuous. If both the hydrogen siloxane fluid and the olefinic compound are added only in part initially, and then all components added continuously after the reaction is initiated, and added until the reactor is full, the reaction is called (correctly) semi-continuous.
Truly continuous reaction of hydrogen siloxane fluid and olefin has, heretofore, not been successfully accomplished. This is for several reasons, which will be enumerated in detail.
There are two main types of continuous reactors for liquid phase systems: continuous stirred tank reactors (known as CSTRs); and plug-flow reactors. In CSTRs, it is inherent that not all of any of the reactants can be consumed completely. However, silicone copolymer containing unreacted hydrogen siloxane fluid is unsuitable for making many commercial products. Thus, CSTRs themselves are not good for making silicone copolymer.
The presence of this unreacted hydrogen siloxane fluid exiting a CSTR reaction might be circumvented by the use of a plug flow reactor; however, without the continual mixing of the CSTR the immiscible hydrogen siloxane fluid and olefinic compound will phase-separate very rapidly subsequent to initial mixing, thus causing the reaction to proceed more and more slowly. In fact, the reaction ceases rapidly without ongoing agitation, and then fails to proceed, even upon renewed agitation, which effect is believed to be caused by gradual, irreversible deactivation of the catalyst. Thus, neither of the two standard continuous reactor systems alone is effective for the manufacture of silicone copolymers.